1. Technical Field
This invention relates to a light emitting device and a method of fabricating the same.
2. Background Art
(First Invention, Second Invention)
A light emitting device having the light emitting layer portion composed of (AlxGa1-x)yIn1-yP alloy (where, 0≦x≦1, 0≦y≦1; also referred to as AlGaInP alloy, or more simply as AlGaInP, hereinafter) can be realized as a high-luminance device by adopting a double heterostructure in which a thin AlGaInP active layer is sandwiched by an n-type AlGaInP cladding layer and a p-type AlGaInP cladding layer, both of which having a band gap energy larger than that of the active layer. Recent efforts have also succeeded in realizing a blue light emitting device in which a similar double heterostructure is formed using InxGayAl1-x-yN (where, 0≦x≦1, 0≦y≦1, x+y≦1).
In an exemplary case of an AlGaInP light emitting device, the double heterostructured, light emitting layer portion is formed on an n-type GaAs substrate by hetero-epitaxial growth, in which an n-type GaAs buffer layer, an n-type AlGaInP cladding layer, an AlGaInP active layer, and a p-type AlGaInP cladding layer are sequentially stacked in this order. Current supply to the light emitting layer portion is achieved through a metal electrode formed on the surface of the device. The metal electrode herein serves as a light interceptor, so that it is formed so as to cover only a center portion of the main surface of the light emitting layer portion, so as to extract light from the surrounding non-electrode-forming region.
In this case, it is advantageous to reduce the area of the metal electrode as possible in view of improving the light extraction efficiency, because the area of the light extraction region formed around the electrode can be enlarged. Although some conventional efforts have been directed to increase the amount of light extraction by effectively spreading the current throughout the device through improving shape of the electrode, increase in the electrode area is inevitable in anyway, and this causes a dilemma that the amount of light extraction is restricted instead by decrease in the narrowing of the light extraction area. What is worse, carrier concentration of dopant in the cladding layer, or conductivity, is suppressed somewhat to a lower level so as to optimize radiative recombination of the carriers within the active layer, and this makes the current less likely to be spread in the in-plane direction. This undesirably results in concentration of current density to the electrode-covered region, and reduction in the substantial amount of light extraction from the light extraction region. One known countermeasure relates to a method of forming a current-spreading layer having a raised carrier concentration (dopant concentration) and a low resistivity between the cladding layer and the electrode. The current-spreading layer is formed by the metal organic vapor phase epitaxy (MOVPE) process or the liquid phase epitaxy (LPE) process.
There are other proposals in which a high-conductivity oxide transparent electrode layer (e.g., ITO (indium tin oxide) transparent electrode layer) is formed, in place of the current-spreading layer composed of a compound semiconductor, so as to cover the surface of the light emitting layer portion, disclosed for example in Japanese Laid-Open Patent Publication No. 1-225178, U.S. Pat. No. 5,789,768 and Japanese Laid-Open Patent Publication No. 6-188455.
In the aforementioned light emitting device, current supply to the light emitting layer portion is effected through a bonding pad composed of a metal and disposed on the outermost surface of the device, and an electrode wire bonded to the bonding pad. In an exemplary configuration of a light emitting element using an oxide transparent electrode layer as disclosed in Japanese Laid-Open Patent Publication No. 1-225178, the oxide transparent electrode layer is formed on the light emitting layer portion while placing an extra thin electrode contact layer in between. The oxide transparent electrode layer can ensure a sufficient current-spreading effect even with a small thickness by virtue of its conductivity as large as that of metals. Bonding of the electrode wire to the bonding pad is, however, disadvantageous in that damages ascribable to the bonding are likely to adversely affect the light emitting layer portion, and are likely to cause failures.
On the other hand, in the configuration using the current-spreading layer composed of a compound semiconductor, the current-spreading layer need to be formed to a certain degree of thickness in order to fully spread current in the in-plane direction. Failures will thus become less likely to occur, because the increased thickness of the current-spreading layer makes it possible, to some degree, to absorb influences of the damage ascribable to the wire bonding. The thus-configured, current-spreading layer need to be raised in the dopant concentration in order to ensure the conductivity, and this tends to result in the problems below.
The light emitting device gradually reduces its emission luminance as the current supply is continued. Assuming now that emission luminance decreasing with the elapse of cumulative current supply time is traced, while defining the emission luminance measured immediately after the current supply was started under a constant current as initial luminance, and further assuming that the time until the emission luminance falls to a predetermined limit luminance, or evaluation current supply time is fixed to a constant value (e.g., 1,000 hours), a ratio of emission luminance after the elapse of the evaluation current supply time to the initial luminance (referred to as “device lifetime”, hereinafter) can be used as a proper index for evaluating service life of the device.
In the light emitting device having the current-spreading layer, the dopant concentration of the cladding layer is kept at an appropriate level in the initial stage of the current supply, but consecutive current supply can accelerate diffusion of the dopant atoms, contained at a high concentration in the current-spreading layer, into the cladding layer and active layer due to electrical factors. If the excessive dopant concentration or the accelerated diffusion of the dopant atoms results in formation of lattice defects, an electrical level which can act as a non-radiative recombination center is formed within the active layer, or at the interface between the p-type cladding layer and the active layer. This undesirably reduces probability of radiative recombination, and consequently lower the emission intensity. That is, time-dependent degradation of the emission luminance becomes more likely to degrade, so that the device lifetime also tends to degrade.
The current-spreading layer, tried to be grown by the MOVPE process or the LPE process, raises another problem in that high growth temperatures in these processes make the dopant more likely to diffuse towards the cladding layer side intrinsically expected to have a low dopant concentration. In addition, growth of a thick current-spreading layer needs a longer time and a larger amount of raw materials, and this tends to lower the production efficiency and to increase the cost.
For the case where the oxide transparent electrode layer as disclosed in Japanese Laid-Open Patent Publication No. 1-225178 is used in combination with the current-spreading layer as disclosed in U.S. Pat. No. 5,789,768, and in particular for the case where the oxide transparent electrode layer is configured as an ITO electrode layer, bonding strength between the resultant ITO electrode layer and the current-spreading layer may extremely be lowered depending on species of the compound semiconductor composing the current-spreading layer. Once this situation occurs, a photo lithography process for forming electrodes or the like, or dicing of a wafer for producing device chips, carried out on the device wafer having the ITO electrode layer already formed thereon, is likely to cause exfoliation of the ITO electrode layer, and this directly results in lowering in yield ratio of the product.
(Third Invention)
Our investigations into the back ground art same as those described in the first and second inventions revealed that the transparent conductive layer composed of ITO tends to raise the contact resistance with the compound semiconductor layer on the device side if used in an intact form, and inevitably lowers the emission efficiency due to increased series resistance. For example, Japanese Laid-Open Patent Publication No. 1-225178 proposes a method of reducing the contact resistance by interposing an electrode contact layer, which comprises an InGaAs layer, between the ITO transparent conductive layer and the compound semiconductor layer on the device side so as to face the entire portion of the ITO transparent conductive layer. It is, however, unconditionally necessary, in view of securing a desirable ohmic contact, to configure the electrode contact layer using InGaAs or the like having a small band gap energy, and this inevitably raises a problem that only a small thickness thereof inevitably result in lowered extraction efficiency due to light absorption. Another problems resides in that, in a fabrication process of the device, a bonding pad to which a current supply wire is to be bonded must be formed anyway on the electrode even if the electrode is formed with a transparent material. In this device, the drive voltage tends to concentrate into the formation region of the bonding pad which is composed of a highly-conductive metal, but tends to be scarce in the region surrounding the pad, which serves as the light extraction region, so that the light extraction efficiency becomes more likely to decrease, and an effect of adopting the transparent electrode cannot always be exhibited to a satisfactory degree.